1. Field of the Invention
The present invention relates to a hydrolysis reactor for removal of urea, ammonia and carbon dioxide from a liquid phase comprising urea in aqueous solution.
As known, waste waters resulting from the purification and recovery process of urea produced in a high-pressure and temperature synthesis reactor have a high residual urea content in aqueous solution generally between 500 ppm and 30xe2x80x2000 ppm and as such cannot be freely discharged into the environment because of the severe antipollution laws in force in industrialized countries.
Each urea production plant must therefore provide appropriate equipment capable of removing residual urea from the waste water so as to lower its concentration to a few ppm, preferably less than 10 ppm.
In the field of treatment of waste waters resulting from the urea purification and recovery process, the requirement for making available residual urea hydrolysis reactors, which would allow on the one hand to obtain non-polluting waste waters with a urea content less than 10 ppm and on the other hand to recover the residual urea (in the form of ammonia and carbon dioxide) contained in these waste waters fed to the hydrolysis reactor, is thus strongly felt.
2. Prior Art
In order to meet the above mentioned requirement, vertical hydrolysis reactors in which flows a liquid phase comprising urea in aqueous solution and a gaseous phase comprising high pressure and temperature steam, generally between 15 bar and 30 bar and between 150xc2x0 C. and 250xc2x0 C., have been becoming more widely used.
These reactors contain a plurality of horizontally extending perforated plates.
The perforated plates have the function of facilitating the mutual mixing of the phases to encourage intimate contact, and thus exchange of mass and heat essential for the hydrolysis reaction of the urea in ammonia (NH3) and carbon dioxide (CO2), and for simultaneous extraction of the NH3 and CO2 from the liquid phase to the gaseous phase.
The heat necessary for decomposition of the urea and extraction of the reaction products from the liquid phase is supplied by the steam contained in the gaseous phase.
Canadian patent application CA-A-2 141 886 describes a reactor of this type in which the liquid phase and the gaseous phase are made to flow in co-current from below upwards through a plurality of horizontal perforated plates.
Although advantageous in many ways, the hydrolysis reactor described above exhibits a number of drawbacks, the first of which is that to obtain the desired degree of decomposition of the urea and related extraction from the liquid phase of the NH3 and the CO2 produced, it is necessary to operate with excess of steam to prevent the hydrolysis reaction from reaching the equilibrium before its time and the gaseous solution from becoming saturated with the reaction products already during its passage through the reactor.
As a result, to obtain an aqueous solution with a residual urea content lower than 10 ppm it is necessary to use high quantities of high pressure and temperature steam, with the ensuing high energy and steam consumption and high operation costs.
The technical problem underlying the present invention is to make available a hydrolysis reactor for removal of urea, ammonia and carbon dioxide from a liquid phase comprising urea in aqueous solution, which would permit operation with low steam and energy consumption and low liquid phase with a urea content below 10 ppm.
In accordance with a first embodiment of the present invention, the above mentioned problem is solved by a hydrolysis reactor for removal of urea, ammonia and carbon dioxide from a liquid phase comprising urea in aqueous solution, comprising:
a substantially cylindrical vertical external shell;
a plurality of perforated plates superimposed and extending horizontally and in mutually spaced relationship in said shell;
an inlet opening for said liquid phase arranged proximate to a lower end of said shell;
first means of supplying a first portion of a gaseous phase comprising high pressure and temperature steam, supported in said shell above said liquid phase inlet opening;
an outlet opening for said liquid phase arranged proximate to an upper end of said shell;
an outlet opening for said gaseous phase arranged proximate to the upper end of said shell; and characterized in that it comprises:
a dividing baffle extending horizontally at a preset height in said shell, in which it defines a first and a second respectively lower and upper reaction space;
means of collection and extraction from the shell of said first gaseous phase portion, supported proximate to said dividing baffle in said first reaction space;
second means of feeding a second portion of said gaseous phase comprising high pressure and temperature steam, supported above said dividing baffle in said second reaction space.
Advantageously, in the hydrolysis reactor according to the present invention, the reaction space in the shell is appropriately divided by a dividing baffle in a first and a second reaction space each of which is fed with a respective gaseous phase portion comprising steam.
In this manner it is possible to use efficiently and rationally the high pressure and temperature steam necessary for decomposition of the urea and extraction of the reaction products, so as to obtain, for equal liquid phase purification degree, a substantial reduction of the quantity of steam to be fed into the hydrolysis reactor with respect to prior art reactors.
Indeed, thanks to the present invention, purification of the liquid phase takes place appropriately in two distinct reaction spaces, into each of which is fed the quantity of steam strictly necessary for obtaining a liquid phase outlet from the hydrolysis reactor with a residual urea concentration below 10 ppm.
In particular, extraction from the first reaction space of the gaseous phase now saturated with reaction products and feeding into the second reaction space of a new gaseous phase comprising high pressure and temperature steam, permit hydrolysis also of the last traces of urea contained in the liquid phase as well as recovery of the NH3 and CO2 in the gaseous phase without thereby having to employ excess of steam.
The liquid phase thus purified can be discharged into the environment but can also be advantageously reused as high temperature and pressure water in the urea synthesis plant or for other industrial uses, e.g. as boiler water.
Another advantage of the hydrolysis reactor which is the object of the present invention lies in the fact that, for equal residual urea concentration contained in the outlet liquid phase, the residence time of the liquid phase in the reactor is significantly lower than the residence time in the prior art reactors.
This permits building a hydrolysis reactor with dimensions and investment costs considerably less than those of the prior art.
Particularly advantageous results were found by arranging the dividing baffle at a height between 55% and 80% of the useful height of the shell.
In the description given below and in the following claims, the term xe2x80x9cuseful heightxe2x80x9d, is understood to mean the height of the shell usable for the urea hydrolysis reaction. In this particular case the useful height is defined by the level reached by the liquid phase in the shell.
Preferably, the dividing baffle is arranged at a height between 65% and 75% of the useful shell height.
In this manner, there is obtained with small amounts of high pressure and temperature steam and low operating costs, a concentration of urea in liquid phase in the first reaction space generally between 30 ppm and 70 ppm and in the second reaction space between 0 ppm and 5 ppm.
In accordance with this first embodiment of the present invention, the hydrolysis reactor includes advantageously a dividing baffle extending horizontally for substantially the entire cross section of the shell.
In addition, the collection and extraction means comprise advantageously:
a collection chamber for the first gaseous phase portion, formed in the first reaction space between the dividing baffle and an inner wall of the shell;
a duct extending coaxially in the shell between the collection chamber and the gaseous phase outlet opening, for extraction from the first reaction space of a two-phase gas/liquid flow.
As a result, practical implementation of the present invention is simple in construction and of low realization cost.
Advantageously, the hydrolysis reactor comprises in addition a gas/liquid separator placed between the extraction duct and the gaseous phase outlet opening, so that the liquid phase if any entrained in the gaseous phase leaving the reactor can be recycled to the reactor and thus permit recovery of the urea contained therein.
In accordance with a second embodiment of the present invention, the above mentioned problem is also solved by a hydrolysis reactor for removal of urea, ammonia and carbon dioxide from a liquid phase comprising urea in aqueous solution, comprising:
a substantially cylindrical vertical external shell;
a plurality of perforated superimposed plates extending horizontal and in mutually spaced relationship in the shell;
an inlet opening of said liquid phase arranged proximate to a lower end of said shell;
first means for feeding a first portion of a gaseous phase comprising high pressure and temperature steam, supported in said shell above said liquid phase inlet opening;
an outlet opening for said gaseous phase arranged proximate to an upper end of the shell; and characterized in that it comprises:
a dividing baffle extending horizontally in the shell at a preset height and for substantially the entire cross section of the shell, with the baffle defining in said shell a first and a second respectively lower and upper reaction space;
a collection chamber for said first gaseous phase portion, formed in said first reaction space between said dividing baffle and an inner wall of said shell;
a duct extending coaxially in the shell between said collection chamber and said gaseous phase outlet opening, for extraction from said first reaction space of a two-phase gas/liquid flow;
second means of feeding a second portion of said gaseous phase comprising high pressure and temperature steam, supported above said dividing baffle in said second reaction space;
an outlet opening for said liquid phase arranged in said second reaction space proximate to and above said dividing baffle.
Advantageously, in this embodiment of the present invention the liquid phase and the gaseous phase are made to flow in counter-current in the second reaction space.
In this manner, it is possible to further improve the mixing of the phases and hence mass and heat exchange so as to facilitate the urea hydrolysis reaction and absorption of the reaction products by the steam.
The characteristics and advantages of the hydrolysis reactor according to the invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings.